This invention relates to rotary drill bits for drilling oil wells and the like, and more particularly to an improved hydraulic action of drilling fluid against the roller cutters of the drill bit and the formation being drilled.
While conventional drill bits have been satisfactory for drilling relatively brittle formations, they do not provide satisfactory rates of penetration when drilling relatively plastically deformable formations. Many commonly encountered formations such as salts, shales, limestones, sandstones and chalks, become plastically deformable under so-called differential pressure conditions, when the hydrostatic pressure of the column of drilling fluid bearing on the bottom of the well bore exceeds the pressure of the formation surrounding the bore, as often occurs in deep hole drilling. Whereas, brittle formations tend to crack or fracture when engaged by the cutting elements of a drill bit, these plastic formations tend to deform and thus remain intact under such loads. In addition, certain of these plastic formations tend to form a relatively thick coating of drilling debris on the roller cutter which can result in so-called "bit-balling" and limited penetration of the formation by the cutting elements.
Attempts have been made to increase the rate of penetration in plastic formations by using extended nozzles for improving the cleaning action of the hydraulic system. While this nozzle arrangement may offer some measure of improved cleaning action, this action is still not satisfactory for many types of plastic formations. Moreover, in this arrangement, the nozzles extend down to points closely adjacent to the bottom of the well bore, and thus are subject to damage by irregularities, such as projections or ridges, on the bottom of the well bore, which may form from time to time during drilling operations.
One area of the formation, particularly in so-called "sticky" formations, tends to adhere abnormally to the gage row of cutting elements on the roller cutter which is the outermost row that determines the diameter or gage of the bore hole. The cutting elements of the gage row normally engages the bore hole bottom at the juncture of the horizontal bottom portion and the cylindrical side portion. Thus, the cutting elements of the gage row are normally engaging the portion of the formation defined by two separate formation surfaces generally at right angles to each other. As a result it has been found that oftentimes an increased amount of so-called sticky formations tends to adhere to the gage row of cutting elements.